Within hours of the sighting on June 5, Frank van Manen caught word that there was a grizzly sow with five cubs in tow spotted in Yellowstone National Park. 

Five cubs following mom is so unlikely that van Manen, who leads the Interagency Grizzly Bear Study Team, pondered an explanation. 

Maybe, he said, there was an adoption event: Sometimes two female siblings produce cubs in the same year, and one ends up with the other’s youngsters. 

“We’ve seen that before, with adoptions taking place,” van Manen said.

Grizzly bears in the Greater Yellowstone Ecosystem typically have one to three cubs. A four-cub litter is an extreme rarity: documented just 11 times since grizzly monitoring began a half-century ago — most recently by world-famous Grizzly 399 in 2020.

“If this was truly a litter of five, it would be the first one recorded in the history of the Greater Yellowstone Ecosystem, for sure,” said van Manen, who leads the Interagency Grizzly Bear Study Team.

The sighting will go down as a record five-cub litter, unless Yellowstone officials somehow come up with genetic samples from all five cubs and can prove one isn’t a sibling, he said.

Last fall, Jackson resident Tim Mayo documented a five-cub black bear litter on Teton Pass, a record for Wyoming.

With grizzlies, a five-cub litter is an exceptional rarity even when looking more broadly across North American grizzly bear range. There have only been “several observations” in total, van Manen said.

There’s been a single observation of a six-cub litter in Alaska, he said, though it may have been the product of an adoption event. 

The massive grizzly brood turned heads in Yellowstone last week. 

West Yellowstone, Montana resident Carolyn Golba, who leads tours in Yellowstone, was in the Lamar Valley area around 8:45 a.m. on her day off when she started to hear chatter on her handheld radio. 

“Somebody kept saying, ‘Five cubs, five cubs!’” Golba recalled. 

The Wild Serenity Wildlife Tour owner booked it toward the Little America region of the park, where the six-bear family was seen. Plentiful tourists, tour guides, and a “lot of bear and wolf people” joined her at the scene. 

At times, she said, there were a dozen bears in view: the sow with five cubs, another sow grizzly with three cubs and two solo black bears. 

“I had 20 grizzlies today — a 20-grizzly day,” Golba said.

It was a personal record for the longtime tour guide.

As for the Greater Yellowstone Ecosystem’s new record-big grizzly bear litter, van Manen admitted his awe. 

“Whether it was an adoption or whether it truly is a five-cub litter, it just amazes me that every year there’s some new surprise to us, even after intensively studying this population for more than 50 years,” van Manen said. “I just think that’s cool.”